Leverage Git servers that enable allowReachableSha1InWant for custom builds instead of always doing a full fetch

Problem Definition

Running a custom build in Bamboo with a specific revision will always perform a full fetch. Since Git 2.5, Git servers can enable uploadpack.allowReachableSha1InWant to allow fetching from a reachable SHA1.

Bitbucket Server supports it since 5.5.

Suggested Solution

We could cut down checkout times for custom builds if we tried to fetch from the specific SHA1 and only falling back to a full fetch if it fails.